Unexpected increase in microalgal removal of doxylamine induced by bicarbonate addition: synergistic chem-/bio-degradation mechanisms

Abstract

Microalgae-based approaches serves as promising methods for remediation of pharmaceutical contaminants (PCs) compare to conventional wastewater treatment processes. However, how to decrease hydraulic retention times of the microalgal system currently has been one of the main bottlenecks. This study constructed an unexpected synergistic extra-chem-/intra-bio-degradation system by addition of 5.95 mM bicarbonate in the microalgal system, which achieved complete removal (100%) of a representative PC, doxylamine (DOX) in 96 h compare to that 192 h in the control. Removal capacities and mass balance analyses demonstrated that biodegradation rate per unit microalgal density was significantly increased by 207%. Further analyses using transcriptomic, enzymatic inhibitory test, and high-resolution mass spectrometry revealed that after addition of bicarbonate for metabolism of DOX, a hydrolase (CYP97C1) and a primary amine oxidase (TynA) can catalyze DOX into doxylamine N-oxide and an intermediate (C15H17NO2) with a m/z of 244.1335. Meanwhile, bicarbonate reacted with microalgae-excreted hydrogen peroxide to form more oxidative radicals including superoxide and hydroxyl radicals extracellularly, which promised the extracellular degradation of DOX according to the oxidative radical inhibiting tests. Further investigation showed addition of bicarbonate in microalgal system improved the removal rate of 17 PCs by up to 500.8%. Therefore, this study not only developed an approach to enhance treatment efficiencies of diverse PCs by microalgae within a shorter time, but also carried unique mechanistic insights into the underlying principles.